1. Field of the Invention
The present invention relates to communications networks in general and more particularly to Fiber Distributed Data Interface (FDDI) networks.
2. Prior Art
The American National Standards Institute (ANSI) has defined standards for a high speed [approximately 100 megabits per sec (MB/sec)] token passing Local Area Network (LAN) known as Fiber Distribution Data Interface (FDDI) network. Because of the high data rate, FDDI is well suited to serve as a backbone or a server network for other lower speed networks (e.g., the IEEE 802.5 Token Ring LAN). In addition, as more demand for higher data rates is placed on conventional lower speed LANS, such networks are unable to provide timely service and higher speed network such as FDDI is required.
Several prior art literature, including ANSI.RTM. X3T9.5, X.sub.3.139, X3.148 and an article entitled "FDDI=The next Generation LAN" by J. Scott Haugdahl and Carl. R. Manson (LAN Technology, pg. 50, Oct. 1989), have given excellent descriptions of FDDI. The named literature is incorporated herein for purposes of background information. Basically, FDDI is a token ring network in which optical fiber and other optical components are used to form the communications media. The network includes two counter-rotating 100 Mbit/sec token passing rings. The rings consist of point-to-point interconnection between neighboring nodes. Even though a FDDI network can be configured in a variety of topologies, a typical topology uses the FDDI network as a backbone. The nodes that are connected to the optical fiber may include data terminal equipment (DTE) such as PBXs, Gateways, CPUs, Wiring Concentrators, etc. Lower data rate networks, utilization devices, etc., can be connected to the DTE. For example, the gateway may attach lower speed networks such as the IEEE 802.5, 802.4 or 802.3 LANs. The concentrator may attach devices such as workstations, PCs, printers, etc. Finally, the PBX may attach telephones. The present invention (to be described subsequently) sets forth an apparatus and method for improving the concentrator.
Data on the rings is repeated by the nodes. The rings are designated as Primary and Secondary. Although data can be transmitted on either ring, normal transmission occurs on the primary ring and the secondary ring is used as a backup in the event of a link or station failure. In the event of such a failure, the entire ring is not brought down. Instead, the failed entity is determined and the loop is reconfigured to bypass the failed entity. Even with the reconfiguration, the overall data rate of the FDDI network is not reduced. Thus, the counter-rotating dual ring design increases the reliability of the FDDI network.
As stated above, the concentrator is a specialized node that attaches to the ring and provides a plurality of ports to which other devices can be attached. Usually, the devices are connected in a starlike configuration. Each device is connected via appropriate communications media to a Concentrator Configuration Element (CCE). The CCE includes a switching mechanism which allows a device to enter (insert) or leave (de-insert from the network. Because the insert/de-insert procedure requires a physical switching of the FDDI media unless devices are inserted/de-inserted in a controlled manner, data on the FDDI media could be corrupted.
An obvious method to compensate for the data corruption is to force the higher layers of the communications architecture into a recovery procedure every time it detects data corruption due to device insertion or de-insertion on the network. The FDDI network conforms to the Open Systems Interconnection (OSI) model defined by the International Standards Organization. Under this model recovery, procedures are handled at the logical link control (LLC) layer. At this level, sophisticated software is required to recover the corrupted data. In addition, several messages have to be exchanged between the receiving device and sending device in order to recover the corrupted data. Therefore, the recovery process, at the LLC layer, is not an optimal solution in that it is wasteful of network bandwidth and places an additional burden on the link control layer.